When working with Java, JAR (Java ARchive) files are commonly used for packaging and distributing Java applications, libraries, or modules. JAR files contain compiled Java bytecode, resources, and metadata. Here’s an overview of compiling with JAR files and creating JAR files, including the commands involved:

Compiling with JAR Files:

To compile Java source code that depends on external JAR files, you need to include the JAR files in the classpath during compilation. Use the -cp or -classpath option followed by the JAR file paths or directories containing JAR files.

javac -cp path/to/external.jar MyFile.java

After compilation, when running the compiled Java program, you also need to include the required JAR files in the classpath using the -cp or -classpath option.

java -cp path/to/external.jar:. MyFile

Creating JAR Files:

Creating JAR Files: To create a JAR file from compiled Java bytecode, you can use the jar command-line tool bundled with the JDK. The basic syntax to create a JAR file is:

jar cvf jar-file input-file(s)

• c: Creates a new JAR file
• v: Verbose output, displays the files being added to the JAR
• f: Specifies the JAR file name

For example, to create a JAR file named “myapp.jar” containing compiled class files in the current directory:

jar cvf myapp.jar *.class

Specifying Main Class:

If your JAR file includes a main class, you need to specify it in the manifest file. Create a text file named Manifest.txt and add the following line:

Main-Class: com.example.MyMainClass

Creating the JAR file with Manifest

Compile the Java source code and place the compiled .class files in a separate directory, such as bin/, using the -d option:

javac -d bin/ src/com/example/MyMainClass.java

Navigate to the bin/ directory, where the compiled .class files are located:
cd bin/
Create the JAR file myapp.jar using the manifest file and the compiled .class files:

jar cvfm ../myapp.jar ../Manifest.txt *

The ../ is used to navigate back to the project-folder/ directory to access the manifest file. The * symbol specifies that all the files in the current directory (bin/) should be included in the JAR file.

Executing the JAR file

To execute the JAR file:
Navigate back to the project-folder/ directory:
cd ..
Run the JAR file using the java -jar command and specify the JAR file’s path and name:

java -jar myapp.jar